The field of the invention is radiotherapy and, in particular, the invention relates to a system and method for accurately performing dose verification for subjects receiving radiation therapy.
External beam radiation therapy is designed to selectively destroy tumor tissue by administering large, spatially-controlled doses of radiation to a subject. The “Rule of Thumb” for such procedures is that the dose delivered should be within ±5 percent of the planned dose and within ±5 mm of the planned position. The treatment process proceeds through a number of steps, beginning with a contoured dose prescription indicated by a radiation oncologist using a set of diagnostic images. A dosimetrist, with the aid of a treatment planning system (TPS), then determines the dose to be delivered from each of a set of beam geometries and incident angles. The TPS utilizes stored dosimetric information, which is typically obtained from measurements on phantoms, to deterministically calculate dose delivery. Once the treatment plan has been approved by the oncologist, the treatment regiment begins. Prior to radiation delivery, the subject is positioned as exactly as possible to match the position used for treatment planning. This includes the alignment of skin markers with room lasers and the acquisition of CT or x-ray images for registration with planning images using either intrinsic or extrinsic fiducial markers. Typically, kilovoltage imaging is performed using an on-board imaging device (OBI) or megavoltage imaging is performed using an electronic portal imaging device (EPID). Immobilization devices can also be used to further increase positioning accuracy and minimize movement during treatment. After proper measures are taken to ensure a subject accurately receives the planned treatment, the radiation dose is delivered, typically at a rate of approximately 400 to 600 cGy per minute.
In radiotherapy, a number of surrogates for determining actual dose delivery are used, some of which are implemented prior to treatment, some during, and some after. Specifically, careful planning and equipment quality assurance provide the basis for determining whether the dose that will be delivered is within ±5 percent of the planned dose and within ±5 mm of the planned position. For example, in one method for performing dose verification, the completed treatment plan can be applied to a tissue or water phantom and the dose may be measured inside the phantom using ion chambers or film. These measurements are then compared with point measurements in the treatment plan to ensure accuracy. Alternatively, EPID images acquired during the phantom irradiation can be compared with digitally reconstructed radiographs (DRRs) generated by the TPS.
Another method for performing dose verification includes placing diodes on the subject's skin to measure skin dose during treatment. Similarly, fiducial markers containing thermoluminescent dosimeters (TLDs) may be implanted in the tumor to measure dose at a number of points. More recently, an implantable MOSFET detector capable of transmitting absorbed dose data to an external handheld reader has been developed, though currently this technology is not widely employed. While skin diodes are non-invasive and provide instantaneous readings, the implanted devices require at least one invasive procedure and can be read only after dose has been delivered. The use of such devices generally adds another step to treatment preparation and reduces treatment efficiency.
Some work has been done to attempt to determine the delivered dose via EPID images obtained during treatment. Such approaches focus on reconstructing the photon fluence at the point of entry by correcting the fluence measured at the EPID for subject attenuation. The calculated entrance fluence is then used in a dose calculation algorithm, which is essentially another TPS, to “reconstruct” an estimate of the dose delivered to the subject.
All of these methods for performing dose verification suffer from the fact that they rely on indirect measurements for dose delivery once dose delivery is substantially occurred and they, accordingly, include inaccuracies associated with the indirect measurements or the ability to correlate actual dose delivery from the indirect measurement. Also, none of these dose verification methods provides three-dimensional measurements of delivered dose.
It would therefore be desirable to have a system and method for accurately quantifying the three-dimensional distribution of radiation dose in subjects receiving radiation therapy.